Prevent duplicate payment transactions with webhook, Postgres, and HTTP API

Description

Quick overview
This workflow exposes a payment webhook that uses Postgres as an idempotency store to prevent duplicate charges, calling an external payment provider API only once per Idempotency-Key and replaying the stored result for retries, mismatches, and in-progress requests.

How it works
Receives a POST request on a webhook endpoint with an Idempotency-Key header and a JSON payment payload.
Ensures the required Postgres idempotency_keys table and indexes exist to track request status and stored responses.
Validates the request, scopes the Idempotency-Key by client ID, and computes a SHA-256 fingerprint of a canonicalized payload.
Atomically claims the idempotency key in Postgres (or reads the existing record) to decide whether to execute, replay a completed response, reject a key reused with different payload, or return an “in progress”/duplicate-order error.
For newly claimed keys, calls the configured payment provider endpoint via HTTP and forwards the same Idempotency-Key header.
Classifies the provider response as completed (replayable success/decline) or failed (retryable timeout/5xx/429), stores the outcome back to Postgres, and responds to the webhook with Idempotency-Key and Idempotent-Replayed headers.

Setup
Add Postgres credentials and ensure the database user has permissions to create tables/indexes and read/write the idempotency_keys table.
Update the providerUrl (and any policy values like leaseSeconds, providerTimeoutMs, maxAmount, and allowedCurrencies) in the validation/fingerprinting code to match your payment provider.
Activate the workflow, copy the production webhook URL, and configure your client or upstream service to send POST requests with an Idempotency-Key header (and optional X-Client-Id) plus the required fields in the JSON body.